Cycles GPU rendering is super slow with bigger image size
You have enough capacity on your tech specs to accelerate the render.
I wouldn´t trust the speed of 2.8 just yet. Instead let´s focus on blender 2.79. Since you don´t make any mention what is exactly what you´re rendering I´d assume it´s a lot of hair/grass or either transparency with lots of bounces.

Bounces on the WORLD are one of those things that can hog up your memory pretty quick. Reduce the bounces on the world to 12 (first). Also on Render settings: Samples render: 24. Lightpath: Bounces: 4. Transmission: 1 if you´re not using any crystal surface you don´t need this). Uncheck reflective caustics and refractive caustics.

Make your 4k image render size: 3840x2160 pixels.
Performance tab (render): uncheck use Hair BVH and set tiles to HILBERT SPIRAL and set the tile size 32x32 (I know, crazy -right?)

AND THIS IS WHERE THE MAGIC HAPPENS:

Create a unique material on the scene assign it to a sphere.

Select SCENE TAB on the properties, and use the material as a global override on the scene (we do this so you don´t have to switch on and off every material on your scene). And hit render.

With a simple diffuse material and your tech specs, your 4K render should be ready in about 2 or 3 minutes (maybe less).

If this is your case, you now know that the slugness is coming from a material rather than the scene setup (of course you can up raise your sample settings all of that) but now you know where to debug.

On the contrary, if your render delayed more than 10 minutes (with material override) it means the polycount on your scene is too large and the render is using disc space as temporal storage data when calculating, hence "fill" and "deploy" until next tile is calculated.

If you make these test and still do not get any variant results, please edit your questions and write more details about what exactly are you doing with the materials and the models.